Verizon has signaled that Yahoo's massive data breach may be enough reason to halt its US$4.8 billion deal to buy the internet company.

On Thursday, Verizon's general counsel Craig Silliman said the company has a "reasonable basis" to believe that the breach involving 500 million Yahoo accounts has had a material impact on the acquisition. This could give the company room to back out or get a large discount.

"We're looking to Yahoo to demonstrate to us the full impact," he added. "If they believe that it's not, then they'll need to show us that."

In response, Yahoo said, "We are confident in Yahoo’s value and we continue to work towards integration with Verizon."

Verizon has declined to comment beyond Silliman's statement. Reportedly, the company was seeking a $1 billion discount on its purchase of Yahoo, claiming that the internet firm had diminished in value since the data breach.

The breach, which Yahoo announced in September, occurred back in 2014. Account details including email addresses, hashed passwords, and phone numbers may have been stolen as result. However, Verizon only learned about the hacking incident two days before Yahoo publicly disclosed it.

On Monday, Verizon's CEO Lowell McAdam reportedly said despite the breach, his company still sees Yahoo as a valuable asset.

“The industrial logic to doing this merger still makes a ton of sense,” he said, according to the Wall Street Journal. “I have spent a lot of time over the past weeks with folks from Yahoo, and I am very impressed by their capability.”

Yahoo has blamed the breach on a state-sponsored actor, without naming the country involved. The FBI is also investigating, and law enforcement officials believe the hacking is consistent with that of nation states.